It's to do with order of inclusion (list order).

The config snippets will be iterated over one at a time to produce the full set of configuration options for the application.

By adding the numbers at the beginning of each filename it allows shell scripts (and other types of programs) to easily grab the directory listing and then process the scripts in the order you want.

Some other examples:

/etc/apt/apt.conf.d/

/etc/systcl.d/

/etc/rc*.d/

/etc/fonts/conf.avail/

/etc/fonts/conf.d/

/etc/udev/rules.d/

For some daemons the file permissions and/or the file extension may also determine if the file is included.